A blood processing apparatus (1) comprises a measurement device (8) having at least one chamber element (80, 81) for receiving a blood fluid, wherein the at least one chamber element (80, 81) extends along a longitudinal axis (L) and comprises a circumferential wall (804, 814) extending about the longitudinal axis (L), a bottom wall (803, 813) and a top wall (805, 815) together defining a flow chamber (802, 812), the at last one chamber element (80, 81) further comprising an inlet port (800, 810) for allowing a flow of a blood fluid into the flow chamber (802, 812) and an outlet port (801, 811) for allowing a flow of a blood fluid out of the flow chamber (802, 812). The blood processing apparatus (1) further comprises a holder device (9) for holding the measurement device (8), the holder device (9) comprising a base (90) having a reception opening (900) for receiving the measurement device (8) and a closure element (91) movably arranged on the base (90) for locking the measurement device (8) in an inserted position in the reception opening (900). An ultrasonic sensor element (92, 93) of the holder device (9) is arranged on the base (90) and adapted to produce an ultrasonic sensor signal (P) for measuring a haematocrit value of a blood fluid in the flow chamber (802, 812). Herein, the ultrasonic sensor element (92, 93), in the inserted position of the measurement device (8), faces the bottom wall (803, 813) of the at least one chamber element (80, 81) for transmitting the ultrasonic signal (P) into the flow chamber (802, 812) through the bottom wall (803, 813). In this way a blood processing apparatus comprising a holder device for a measurement device is provided which allows to easily insert the measurement device into the holder device and allows for a reliable measurement of, in particular, a haematocrit value of a blood flow through the measurement device.
